This invention relates to image sensors of the kind utilising charge coupled devices (CCD's), and an improved method of operation.
In CCD frame transfer image sensors, light representing an image is focussed onto a first photosensitive portion of an array of CCD channels comprising an image section of the array. A second portion of the array of CCD channels which is non photosensitive, (e.g, due to being shielded from light), comprises a store section Adjacent to the store section there is provided a read-out section A frame of charge photogenerated in the image section is transferred to the store section by applying clock pulses to the control electrodes of both sections, then read out in sequential line-by-line fashion by applying clock pulses to the control electrodes of both store and line read-out sections. Normally a frame of charge is read out of the store section as a further frame is being collected in the image section, and so on. Thus the `integration period` during which a frame of photogenerated charge is collected in the image section of the sensor is usually about the same as the time taken to read out the previous frame of charge from the store section. The read out time is often fixed to conform to a given output standard, for example as in conventional 525/625 line television and is, in any case, limited by the physical constraints of the device structure, for example the speed of charge transfer.
This relatively long integration period can result in image smearing if the image sensor is used to image a fast moving object. With a conventional television camera or a CCD image sensor, a bright flash of illumination can be used to freeze the motion of the image after the manner of a stroboscope, but such an arrangement is inconvenient and cumbersome. It has been proposed to delay the onset of the integration period of the CCD sensor until immediately prior to the commencement of the read-out period, but of course this results in the short integration period occurring at a fixed predetermined instant in relation to the frame period. Whilst for some applications this may not be a drawback, it can cause a serious problem when a briefly occurring event is to be captured by the image sensor, as unless the event happens to occur during the very brief integration period, the event will be missed altogether and go unrecorded by the image sensor. The present invention seeks to reduce this difficulty.